Influences of oceanic rheostats and amplifiers on atmospheric CO2 content during the Late Quaternary

Although a clear explanation has yet to emerge for the observed decline in atmospheric CO2 content during glacial episodes, numerous hypotheses have b een offered. Selected examples are reviewed or evaluated here. A strengthen ed biological pump resulting from intensified upwelling at low latitudes wa s suggested early on as a likely direct cause of the glacial CO2 drawdown. Two problems have since arisen with this hypothesis. First, on some upwelli ng continental margins, for example off California, Oregon, northwestern Me xico, Peru, and parts of NW Africa, organic matter accumulation decreased, in some areas markedly, during the Last Glacial Maximum (LGM). Second, upwe lled cool CO2-rich water degasses carbon dioxide in warm regions, and for a key area such as the modern equatorial Pacific to become a net sink for at mospheric CO2, rather than a source as it is today, the rate of downward or ganic carbon export must exceed the rate of degassing. Recent sedimentary c arbon and nitrogen isotopic data suggest however that the equatorial Pacifi c has remained a strong CO2 source for at least the last 40,000 yr and prob ably much longer. Although the direct extraction of carbon from surface wat ers and burial in the sediments in productive regions now appears unlikely to provide the sought-for explanation, indirect effects related to export p roductivity may hold the key. These can modulate the chemical character of the ocean so as to increase its uptake of CO2 (a rheostat effect) or they c ould increase indirectly the nutrient inventory of the sea (an amplifier ef fect). For example, it is now recognized that denitrification was greatly r educed during glacial maxima in all three principal oxygen minimum zones in the oceans, i.e. the subtropical north and south Pacific and the Arabian S ea. The implication is that nitrate, the limiting nutrient in the modern oc ean, must have been more abundant during glacial periods, and that this sur feit would have supported increased export production in the meso or oligot rophic areas that are today nitrate-limited. Dust may represent another rhe ostat. Increases in atmospheric turbidity during glacials are clearly recor ded by ice and marine sediment cores in both hemispheres. In addition to fo stering enhanced export production by adding needed iron to nitrate-rich ar eas, it has been suggested recently that increased dust inputs to nitrate-d epleted regions during glacials might have encouraged growth of iron-hungry N-2 fixing cyanobacteria, thus alleviating nitrate limitation. No sediment ary delta(15)N data yet exist to support this hypothesis, but it remains vi able. Finally, regional changes in physical hydrography may have played a m ajor and hitherto underestimated role. For example, in the glacial Southern Ocean south of the Polar Front, recent nitrogen isotope and other data imp ly that the upper water column was well stratified during the LGM. By limit ing upwelling, this would have reduced the ocean-atmosphere CO2 'leak' in t he area. This could have made a significant contribution to the pCO(2) draw down. (C) 1999 Elsevier Science Ltd. All rights reserved.